docs: def add(self, stuff, sloppy=False):
          """Add variables and constraints.
  
          Parameters
          ----------
          stuff : iterable, Variable, Constraint
              Either an iterable containing variables and constraints or a single variable or constraint.
  
          sloppy : bool
              Check constraints for variables that are not part of the model yet.
  
          Returns
          -------
          None
          """
          if self._pending_modifications.toggle == 'remove':
              self.update()
              self._pending_modifications.toggle = 'add'
          if isinstance(stuff, collections.Iterable):
              for elem in stuff:
                  self.add(elem, sloppy=sloppy)
          elif isinstance(stuff, Variable):
              if stuff.__module__ != self.__module__:
                  raise TypeError("Cannot add Variable %s of interface type %s to model of type %s." % (
                      stuff, stuff.__module__, self.__module__))
              self._pending_modifications.add_var.append(stuff)
          elif isinstance(stuff, Constraint):
              if stuff.__module__ != self.__module__:
                  raise TypeError("Cannot add Constraint %s of interface type %s to model of type %s." % (
                      stuff, stuff.__module__, self.__module__))
              if sloppy is True:
                  self._pending_modifications.add_constr_sloppy.append(stuff)
              else:
                  self._pending_modifications.add_constr.append(stuff)
          else:
              raise TypeError("Cannot add %s. It is neither a Variable, or Constraint." % stuff)